Submerged arc welding is a fully mechanised welding method characterised by high productivity and quality, often used for longer welding seems in thicker materials. During submerged arc welding one or more sequentially arranged welding electrodes melt in arcs.
The weld, particularly the melted material and the arc, are protected beneath a layer of pulverised flux. The flux melts in part during the process, thus creating a protecting layer of slag on the weld pool. The electrical current used in the process is relatively high, usually within 300-1500 Ampere per electrode. The electrodes used in submerged arc welding are usually 2.5-6 mm in diameter.
Fluxes used in submerged arc welding are granular fusible minerals typically containing oxides of manganese, silicon, titanium, aluminium, calcium, zirconium, magnesium and other compounds such as calcium fluoride. The flux is specially formulated to be compatible with a given electrode wire type so that the combination of flux and wire yields desired mechanical properties. All fluxes react with the weld pool to produce the weld metal chemical composition and mechanical properties. It is common practice to refer to fluxes as ‘active’ if they add manganese and silicon to the weld, the amount of manganese and silicon added is influenced by the arc voltage and the welding current level.
To find the highest productivity possible with submerged arc welding, with increased competitiveness as one result, one strives for increased weld speed and the highest possible deposition rate, i.e. melted welding consumables, or really created joint material, per hour and electrode.
When welding with a single electrode, as opposed to multiple sequential electrodes, the upper limit is often reached, making further improvements in the welding productivity impossible by only changing the weld data. For instance, when increasing the weld current the arc finally becomes strong enough to push the weld pool resulting in unacceptable welds.
One solution to this known in the art is to use multiple electrodes, positioned sequentially in the direction of the weld seem. Usually 2-3 electrodes are used, however, usage of up to 6 electrodes is known.
Unfortunately a multiple electrode set-up is not problem-free as the individual arcs affect each other through so called “magnetic arc blow effect”. This effect is caused by magnetic fields generated by the current flowing through adjacent electrodes. The “magnetic arc blow effect” affects an adjacent arc, making it deviate or deflect from the usual and wanted direction, which is in most cases perpendicular to the material and in line with the electrode. This deviation can cause the arc(s) to push the weld pool in an unfavourable way resulting in a wave-form weld and unacceptable overall results.
Furthermore the molten material in the weld pool is influenced by forces from the arcs forming a sensitive system that affects the pattern of waves in the weld pool. Fluid material is squeezed between the arcs so that the whole weld pool can be seen as a connected system of n−1 weld pools, n being the number of arcs.
To inhibit this phenomenon one known solution is to power the primer electrode with DC current while powering the sequential ones with AC current. Using AC current in these situations has been proven useful for a number of reasons. For instance, a shifting magnetic field does not reach the arc to the same extent, especially for instance in a deep weld joint, as vortexes in the base material inhibit the magnetic field dispersion, also with a directionally fluctuating magnetic field the arc deviations are no longer mono-directional, resulting in less impact on the weld pool. A further benefit with AC current on the sequential electrodes is an increased deposition rate.
Even though the above mentioned solutions increase the weld speed and deposition rate there appears to be an upper limit hard to surpass without jeopardising the quality of the welding result.
Pushing beyond the limit can cause instability in the welding process at the latter electrodes. This is expected to depend on the relatively larger weld pool found at this position caused by the melted consumables from the leading electrodes in combination with the push effect on the weld pool from the arcs. To a small degree this effect can be lessened by sequentially lowered weld currents used to power the latter electrodes, although the problem can not be fully avoided through this measure.